Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270106045550/iz3017sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270106045550/iz3017Isup2.hkl |
For related literature, see: Baran et al. (2006); Baur (1981); Brese & O'Keeffe (1991); Brown & Altermatt (1985); Ferraris (1970); Ferraris & Ivaldi (1984); Kolitsch (2004); Kolitsch & Fleck (2006); Kolitsch & Schwendtner (2004, 2005); Lii & Wu (1994); Schwendtner (2006); Schwendtner & Kolitsch (2004a, 2004b, 2005a, 2005b, 2005c, 2006); Schwendtner et al. (2006a, 2006b); Spek (2003).
The title compound was prepared hydrothermally (Teflon-lined stainless steel bomb, 493 K, nine days, slow furnace cooling) from a mixture of Cs2CO3, Al2O3 and H3AsO4·0.5H2O, with a volume ratio of approximately 1:1:3. The Teflon containers were then filled with distilled water to about 80% of their inner volume. The pH value of the starting and final solutions was approximately 1 and 1/2, respectively. CsAl(H2AsO4)2(HAsO4) formed colourless prismatic crystals up to 0.8 mm in length (yield ca 60%) and was acompanied by hexagonal tabular crystals of RbFe(HPO4)2)-type (Lii & Wu, 1994) CsAl(HAsO4)2 (ca 5%) and an uninvestigated amorphous mass (ca 35%) (Schwendtner & Kolitsch, 2005c).
The choice of space group Cc was based on the following evidence: (i) A refinement in space group C2/c and using the model of CsGa(H1.5AsO4)2(H2AsO4) (Schwendtner & Kolitsch, 2005a) ?? resulted in R(F) = 0.15 and high residual electron densities (6–3 e Å-3); application of various probable twin laws did not lead to any improvement. (ii) All intensity statistics showed a distinct preference for non-centrosymmetry, whereas those of CsMIII(H1.5AsO4)2(H2AsO4) (MIII = Ga and Cr; Schwendtner & Kolitsch, 2005a) ?? clearly indicated centrosymmetry. (iii) A search for higher symmetry using PLATON (Spek, 2003) was unsuccessful. (iv) The correlation parameters calculated from the last refinement step were negligible (the largest correlation was between the z and y coordinates of atom H2. (v) The structure model and the inferred hydrogen-bonding scheme (different from that of the C2/c model) are highly plausible. O—H bond lengths for atoms H1, H2, H4 and H5 were restrained to a length of 0.90 (1) Å. The position of atom H3 was taken from a difference Fourier map and fixed. H1 was constrained to have a fixed Uiso(H) value of 0.05 Å2. The crystal was refined as a racemic twin.
Data collection: COLLECT (Nonius, 2003); cell refinement: SCALEPACK (Otwinowski et al., 2003); data reduction: SCALEPACK and DENZO (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: SHELXL97.
CsAl(H2AsO4)2(HAsO4) | F(000) = 1072 |
Mr = 581.69 | Dx = 3.756 Mg m−3 |
Monoclinic, Cc | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: C -2yc | Cell parameters from 2339 reflections |
a = 4.634 (1) Å | θ = 2.0–35.0° |
b = 14.672 (3) Å | µ = 13.32 mm−1 |
c = 15.153 (3) Å | T = 293 K |
β = 93.11 (3)° | Prism, colorless |
V = 1028.7 (4) Å3 | 0.10 × 0.09 × 0.02 mm |
Z = 4 |
Nonius KappaCCD diffractometer | 4456 independent reflections |
Radiation source: fine-focus sealed tube | 4221 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.032 |
ϕ and ω scans | θmax = 35.0°, θmin = 2.7° |
Absorption correction: multi-scan (SCALEPACK; Otwinowski et al., 2003) | h = −7→7 |
Tmin = 0.349, Tmax = 0.777 | k = −23→23 |
4467 measured reflections | l = −24→24 |
Refinement on F2 | Hydrogen site location: difference Fourier map |
Least-squares matrix: full | H atoms treated by a mixture of independent and constrained refinement |
R[F2 > 2σ(F2)] = 0.024 | w = 1/[σ2(Fo2) + (0.0327P)2 + 4.0552P] where P = (Fo2 + 2Fc2)/3 |
wR(F2) = 0.061 | (Δ/σ)max = 0.024 |
S = 1.02 | Δρmax = 0.81 e Å−3 |
4456 reflections | Δρmin = −1.08 e Å−3 |
170 parameters | Extinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
4 restraints | Extinction coefficient: 0.00190 (12) |
Primary atom site location: structure-invariant direct methods | Absolute structure: Flack (1983), 2188 Friedel pairs? |
Secondary atom site location: difference Fourier map | Absolute structure parameter: 0.507 (9) |
CsAl(H2AsO4)2(HAsO4) | V = 1028.7 (4) Å3 |
Mr = 581.69 | Z = 4 |
Monoclinic, Cc | Mo Kα radiation |
a = 4.634 (1) Å | µ = 13.32 mm−1 |
b = 14.672 (3) Å | T = 293 K |
c = 15.153 (3) Å | 0.10 × 0.09 × 0.02 mm |
β = 93.11 (3)° |
Nonius KappaCCD diffractometer | 4456 independent reflections |
Absorption correction: multi-scan (SCALEPACK; Otwinowski et al., 2003) | 4221 reflections with I > 2σ(I) |
Tmin = 0.349, Tmax = 0.777 | Rint = 0.032 |
4467 measured reflections |
R[F2 > 2σ(F2)] = 0.024 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.061 | Δρmax = 0.81 e Å−3 |
S = 1.02 | Δρmin = −1.08 e Å−3 |
4456 reflections | Absolute structure: Flack (1983), 2188 Friedel pairs? |
170 parameters | Absolute structure parameter: 0.507 (9) |
4 restraints |
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. |
x | y | z | Uiso*/Ueq | ||
Cs1 | −0.00002 (7) | −0.009538 (17) | 0.00001 (2) | 0.02820 (7) | |
Al1 | −0.0307 (3) | 0.14086 (6) | 0.24624 (8) | 0.00818 (13) | |
As1 | 0.49219 (5) | 0.21759 (2) | 0.36942 (2) | 0.00885 (6) | |
As2 | 0.47210 (7) | −0.00622 (2) | 0.24955 (3) | 0.00921 (6) | |
As3 | 0.44465 (5) | 0.20973 (2) | 0.11976 (2) | 0.00878 (6) | |
O1 | 0.7498 (5) | 0.14258 (16) | 0.34791 (16) | 0.0107 (4) | |
O2 | 0.2138 (5) | 0.23278 (16) | 0.29550 (17) | 0.0113 (4) | |
O3 | 0.3831 (6) | 0.1974 (2) | 0.47177 (17) | 0.0185 (5) | |
O4 | 0.6303 (6) | 0.32747 (18) | 0.3707 (2) | 0.0224 (6) | |
H4 | 0.7859 | 0.3283 | 0.3992 | 0.034* | |
O5 | 0.7187 (5) | 0.04969 (17) | 0.19569 (17) | 0.0111 (4) | |
O6 | 0.2129 (5) | 0.04928 (17) | 0.30031 (17) | 0.0116 (4) | |
O7 | 0.3175 (7) | −0.07964 (19) | 0.1726 (2) | 0.0198 (5) | |
H7 | 0.2986 | −0.1300 | 0.1951 | 0.030* | |
O8 | 0.6294 (6) | −0.0709 (2) | 0.3336 (2) | 0.0194 (5) | |
H8 | 0.7681 | −0.0985 | 0.3154 | 0.029* | |
O9 | 0.1886 (5) | 0.13530 (17) | 0.14408 (17) | 0.0120 (4) | |
O10 | 0.7244 (5) | 0.23282 (16) | 0.19080 (16) | 0.0108 (4) | |
O11 | 0.5659 (6) | 0.1710 (2) | 0.02259 (19) | 0.0202 (5) | |
H11 | 0.6817 | 0.2076 | 0.0039 | 0.030* | |
O12 | 0.2957 (6) | 0.31461 (18) | 0.0985 (2) | 0.0179 (5) | |
H12 | 0.1507 | 0.3090 | 0.0654 | 0.027* |
U11 | U22 | U33 | U12 | U13 | U23 | |
Cs1 | 0.04127 (16) | 0.02128 (11) | 0.02123 (10) | −0.00028 (11) | −0.00575 (9) | −0.00348 (9) |
Al1 | 0.0062 (3) | 0.0095 (3) | 0.0089 (3) | 0.0002 (3) | 0.0004 (2) | 0.0002 (3) |
As1 | 0.00701 (13) | 0.01019 (12) | 0.00932 (14) | 0.00092 (9) | 0.00010 (10) | −0.00164 (9) |
As2 | 0.00697 (11) | 0.00857 (11) | 0.01210 (12) | 0.00053 (11) | 0.00069 (9) | 0.00054 (11) |
As3 | 0.00777 (13) | 0.01035 (12) | 0.00817 (14) | −0.00140 (9) | −0.00011 (10) | 0.00064 (10) |
O1 | 0.0079 (9) | 0.0130 (10) | 0.0115 (10) | 0.0035 (7) | 0.0030 (8) | 0.0027 (8) |
O2 | 0.0081 (10) | 0.0109 (9) | 0.0145 (11) | 0.0001 (7) | −0.0027 (8) | 0.0007 (8) |
O3 | 0.0179 (12) | 0.0294 (13) | 0.0082 (10) | 0.0044 (10) | 0.0019 (9) | −0.0011 (9) |
O4 | 0.0170 (12) | 0.0117 (11) | 0.0379 (17) | −0.0040 (9) | −0.0035 (11) | −0.0045 (10) |
O5 | 0.0079 (9) | 0.0126 (9) | 0.0132 (10) | −0.0027 (7) | 0.0023 (8) | −0.0005 (8) |
O6 | 0.0095 (10) | 0.0127 (10) | 0.0127 (10) | 0.0031 (7) | 0.0027 (8) | 0.0020 (8) |
O7 | 0.0215 (13) | 0.0128 (10) | 0.0245 (13) | −0.0054 (9) | −0.0044 (10) | −0.0053 (9) |
O8 | 0.0162 (12) | 0.0189 (12) | 0.0228 (13) | 0.0060 (9) | 0.0000 (10) | 0.0089 (10) |
O9 | 0.0077 (10) | 0.0155 (10) | 0.0127 (11) | −0.0032 (7) | 0.0018 (8) | −0.0002 (8) |
O10 | 0.0080 (10) | 0.0116 (9) | 0.0123 (10) | −0.0013 (7) | −0.0037 (8) | 0.0019 (8) |
O11 | 0.0191 (12) | 0.0285 (13) | 0.0138 (11) | −0.0085 (10) | 0.0078 (9) | −0.0047 (10) |
O12 | 0.0167 (12) | 0.0126 (10) | 0.0236 (13) | 0.0010 (8) | −0.0072 (9) | 0.0056 (9) |
Cs1—O7 | 3.107 (3) | Al1—O6 | 1.911 (3) |
Cs1—O9 | 3.137 (3) | Al1—O10iv | 1.926 (3) |
Cs1—O12i | 3.152 (3) | As1—O1 | 1.669 (2) |
Cs1—O1ii | 3.190 (2) | As1—O2 | 1.677 (2) |
Cs1—O8ii | 3.199 (3) | As1—O3 | 1.683 (3) |
Cs1—O6iii | 3.285 (3) | As1—O4 | 1.734 (3) |
Cs1—O3iii | 3.319 (3) | As2—O5 | 1.657 (2) |
Cs1—O11iv | 3.355 (3) | As2—O6 | 1.673 (2) |
Cs1—O4v | 3.386 (3) | As2—O7 | 1.716 (3) |
Cs1—O5iv | 3.415 (3) | As2—O8 | 1.718 (3) |
Cs1—O11 | 3.730 (3) | As3—O9 | 1.668 (2) |
Al1—O2 | 1.889 (3) | As3—O10 | 1.674 (2) |
Al1—O1iv | 1.892 (3) | As3—O11 | 1.702 (3) |
Al1—O9 | 1.899 (3) | As3—O12 | 1.710 (3) |
Al1—O5iv | 1.905 (3) | ||
O2—Al1—O1iv | 90.35 (12) | O2—As1—O4 | 98.87 (13) |
O2—Al1—O9 | 90.96 (12) | O3—As1—O4 | 106.38 (16) |
O1iv—Al1—O9 | 178.30 (11) | O5—As2—O6 | 121.14 (11) |
O2—Al1—O5iv | 179.04 (12) | O5—As2—O7 | 104.46 (14) |
O1iv—Al1—O5iv | 89.65 (12) | O6—As2—O7 | 109.54 (14) |
O9—Al1—O5iv | 89.06 (12) | O5—As2—O8 | 111.23 (13) |
O2—Al1—O6 | 90.28 (12) | O6—As2—O8 | 102.39 (14) |
O1iv—Al1—O6 | 89.51 (11) | O7—As2—O8 | 107.56 (14) |
O9—Al1—O6 | 89.40 (12) | O9—As3—O10 | 121.71 (12) |
O5iv—Al1—O6 | 90.67 (10) | O9—As3—O11 | 104.18 (13) |
O2—Al1—O10iv | 89.93 (10) | O10—As3—O11 | 110.07 (14) |
O1iv—Al1—O10iv | 90.90 (11) | O9—As3—O12 | 110.25 (13) |
O9—Al1—O10iv | 90.18 (12) | O10—As3—O12 | 103.28 (13) |
O5iv—Al1—O10iv | 89.12 (11) | O11—As3—O12 | 106.61 (15) |
O6—Al1—O10iv | 179.54 (13) | As1—O4—H4 | 109.5 |
O1—As1—O2 | 119.33 (12) | As2—O7—H7 | 109.5 |
O1—As1—O3 | 108.52 (13) | As2—O8—H8 | 109.5 |
O2—As1—O3 | 112.35 (13) | As3—O11—H11 | 109.5 |
O1—As1—O4 | 110.36 (14) | As3—O12—H12 | 109.5 |
Symmetry codes: (i) x−1/2, y−1/2, z; (ii) x−1, −y, z−1/2; (iii) x, −y, z−1/2; (iv) x−1, y, z; (v) x−1/2, −y+1/2, z−1/2. |
D—H···A | D—H | H···A | D···A | D—H···A |
O4—H4···O11vi | 0.82 | 2.22 | 2.978 (4) | 154 |
O7—H7···O10i | 0.82 | 2.04 | 2.801 (4) | 154 |
O8—H8···O4vii | 0.82 | 2.13 | 2.791 (4) | 137 |
O11—H11···O3viii | 0.82 | 1.76 | 2.569 (4) | 169 |
O12—H12···O3v | 0.82 | 1.84 | 2.641 (4) | 167 |
Symmetry codes: (i) x−1/2, y−1/2, z; (v) x−1/2, −y+1/2, z−1/2; (vi) x+1/2, −y+1/2, z+1/2; (vii) x+1/2, y−1/2, z; (viii) x+1/2, −y+1/2, z−1/2. |
Experimental details
Crystal data | |
Chemical formula | CsAl(H2AsO4)2(HAsO4) |
Mr | 581.69 |
Crystal system, space group | Monoclinic, Cc |
Temperature (K) | 293 |
a, b, c (Å) | 4.634 (1), 14.672 (3), 15.153 (3) |
β (°) | 93.11 (3) |
V (Å3) | 1028.7 (4) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 13.32 |
Crystal size (mm) | 0.10 × 0.09 × 0.02 |
Data collection | |
Diffractometer | Nonius KappaCCD diffractometer |
Absorption correction | Multi-scan (SCALEPACK; Otwinowski et al., 2003) |
Tmin, Tmax | 0.349, 0.777 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 4467, 4456, 4221 |
Rint | 0.032 |
(sin θ/λ)max (Å−1) | 0.806 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.024, 0.061, 1.02 |
No. of reflections | 4456 |
No. of parameters | 170 |
No. of restraints | 4 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.81, −1.08 |
Absolute structure | Flack (1983), 2188 Friedel pairs? |
Absolute structure parameter | 0.507 (9) |
Computer programs: COLLECT (Nonius, 2003), SCALEPACK (Otwinowski et al., 2003), SCALEPACK and DENZO (Otwinowski & Minor, 1997), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), DIAMOND (Brandenburg, 2006), SHELXL97.
Cs1—O7 | 3.107 (3) | Al1—O6 | 1.911 (3) |
Cs1—O9 | 3.137 (3) | Al1—O10iv | 1.926 (3) |
Cs1—O12i | 3.152 (3) | As1—O1 | 1.669 (2) |
Cs1—O1ii | 3.190 (2) | As1—O2 | 1.677 (2) |
Cs1—O8ii | 3.199 (3) | As1—O3 | 1.683 (3) |
Cs1—O6iii | 3.285 (3) | As1—O4 | 1.734 (3) |
Cs1—O3iii | 3.319 (3) | As2—O5 | 1.657 (2) |
Cs1—O11iv | 3.355 (3) | As2—O6 | 1.673 (2) |
Cs1—O4v | 3.386 (3) | As2—O7 | 1.716 (3) |
Cs1—O5iv | 3.415 (3) | As2—O8 | 1.718 (3) |
Cs1—O11 | 3.730 (3) | As3—O9 | 1.668 (2) |
Al1—O2 | 1.889 (3) | As3—O10 | 1.674 (2) |
Al1—O1iv | 1.892 (3) | As3—O11 | 1.702 (3) |
Al1—O9 | 1.899 (3) | As3—O12 | 1.710 (3) |
Al1—O5iv | 1.905 (3) |
Symmetry codes: (i) x−1/2, y−1/2, z; (ii) x−1, −y, z−1/2; (iii) x, −y, z−1/2; (iv) x−1, y, z; (v) x−1/2, −y+1/2, z−1/2. |
D—H···A | D—H | H···A | D···A | D—H···A |
O4—H4···O11vi | 0.82 | 2.22 | 2.978 (4) | 154.3 |
O7—H7···O10i | 0.82 | 2.04 | 2.801 (4) | 153.7 |
O8—H8···O4vii | 0.82 | 2.13 | 2.791 (4) | 137.3 |
O11—H11···O3viii | 0.82 | 1.76 | 2.569 (4) | 168.6 |
O12—H12···O3v | 0.82 | 1.84 | 2.641 (4) | 166.9 |
Symmetry codes: (i) x−1/2, y−1/2, z; (v) x−1/2, −y+1/2, z−1/2; (vi) x+1/2, −y+1/2, z+1/2; (vii) x+1/2, y−1/2, z; (viii) x+1/2, −y+1/2, z−1/2. |
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Arsenates, like phosphates and silicates, tend to form tetrahedral–octahedral framework structures with potentially interesting properties, such as ion conductivity, ion exchange and catalytic properties. The system MI—MIII—As—O—H (MI = Li, Na, K, Rb, Cs, Ag, Tl, NH4 and H3O; MIII = Al, Ga, In, Sc, Cr and Fe) is currently being closely studied by the authors to extend the knowledge on possible new arsenates and structure types. A larger number of novel structure types and new compounds has been prepared and structurally characterized so far (Kolitsch, 2004; Kolitsch & Schwendtner, 2004, 2005; Schwendtner & Kolitsch, 2004a,b, 2005a,b, c, 2006; Baran et al., 2006; Schwendtner, 2006; Schwendtner et al., 2006a,b).
The new compound CsAl(H2AsO4)2(HAsO4) is monoclinic (space group Cc) and represents a new chain-based structure type, very closely related to CsGa(H1.5AsO4)2(H2AsO4) and isotypic CsCr(H1.5AsO4)2(H2AsO4) (Schwendtner & Kolitsch, 2005a). The asymmetric unit contains 22 atoms, all of which lie in general positions. Al1 is octahedrally coordinated (mean Al1—O = 1.904 Å), Table 1) and corner-linked to six AsO4 tetrahedra to form decorated kröhnkite-like chains (Kolitsch & Fleck, 2006 and references therein) running parallel to [100] (Fig. 1). Ten-coordinated Cs1 atoms (mean Cs1—O = 3.254 Å) lie between these chains. All three crystallographically different AsO4 tetrahedra are protonated and highly distorted. The As—OH bonds are considerably elongated compared with the As—O bonds, which is typical for protonated AsO4 tetrahedra (Ferraris, 1970; Ferraris & Ivaldi, 1984). The mean As—O bond length in the three AsO4 tetrahedra ranges from 1.689 - 1.690 Å, which is considerably longer than the average length for AsO4 tetrahedra of 1.682 Å (Baur, 1981) and a result of the protonation. As3O4 is doubly protonated and is involved in one strong (O12···O3 = 2.641 (4) Å) and one very strong H-bond (O11···O3 = 2.569 (4) Å). As2O4 is also doubly protonated with two medium-strength H-bonds (O8···O4 = 2.791 (4) and O7···O10 = 2.801 (4) Å), whereas As1O4 is only involved in one weak H-bond (O4···O11 = 2.978 (4) Å). The hydrogen bond between O11 and O3 is strongly disordered between the two O atoms. Bond-valence calculations suggest that both atoms act as donor and as acceptor, since both atoms are highly underbonded (1.35 and 1.25 valence units, for atoms O3 and O11, respectively). The longer As—O bond length to O11 and the smaller bond valence may indicate that the H atom is located on a positon closer to the O11 atom, at least statistically and has therefore been attached to O11 in the refinement process.
Bond-valence sums for all atoms were calculated using the bond-valence parameters from Brese & O'Keeffe (1991) for Al and Brown & Altermatt (1985) for As and Cs. They are 1.08 (Cs1), 3.03 (Al1), 4.93 (As1, As2), 4.94 (As3), 1.95 (O1), 1.80 (O2), 1.35 (O3), 1.17 (O4), 1.91 (O5), 1.89 (O6), 1.30 (O7), 1.27 (O8), 1.96 (O9), 1.76 (O10), 1.25 (O11) and 1.30 (O12) valence units. For the cations they are close to the ideal valences. The low sums for O3, O4, O7, O8, O11 and O12 demonstrate that these belong to OH groups. The hydrogen bonds donated by the OH groups are accepted by O3, O4, O10 and O11 (Table 2).
The similarities between CsAl(H2AsO4)2(HAsO4) and CsMIII(H1.5AsO4)2(H2AsO4) (MIII = Ga, Cr) (C2/c) (Schwendtner & Kolitsch, 2005) are striking. Firstly, they have very similar unit-cell parameters [a = 4.634 (1), 4.714 (1) and 4.744 (1); b = 14.672 (3), 14.674 (3) and 14.625 (3); c = 15.143 (3), 15.162 (3) and 15.127 (3) Å; β = 93.11 (3), 93.31 (3) and 93.48 (3) °, V = 1028.7 (4), 1047.1 (4) and 1047.6 (4) Å3 for the CsAl, CsGa and CsCr compounds, respectively]. Both Cc and C2/c structure types, contain decorated kröhnkite-like chains running parallel [100]. Owing to the smaller size of the Al atom compared wizh Cr or Ga, the structure is distorted, which results in a loss of the twofold axis. Polyhedra As1O4 (1 times protonated) and As3O4 (2 times protonated) in the title compound are crystallographically equivalent to the As1O4 polyhedra (1.5 times protonated) in the CsGa- and CsCr-compounds. The formerly symmetry-restricted split double-well H position in the C2/c-type is not clearly detectable, but has probably now shifted towards As3 in the Cc-type (compare Figs. 1 and 2). Another difference concerns the double protonated As2O4 groups in both structure types. In the C2/c-type As2 is located on a twofold axis and the two H2 atoms bonded to O6 are crystallographically equivalent (Fig. 1 b, 2 b). This is not the case in the Cc-type structure, where atom H7 behaves very similarly to what we see in the C2/c-type, but atom H8 shifts considerably and its hydrogen bond is accepted by a different O atom, O4 (compare Figs. 1a and 1 b). Th Cs atom in the C2/c-type structure is located on the origin (0,0,0), whereas it is slightly offset in direction b in the Cc-type structure [y = -0.009538 (17)] (compare Figs. 1 and 2), which is in concordance with the lower space group symmetry. The ellipsoid drawing of CsAl(H2AsO4)2(HAsO4) is given in Fig. 3.